An electronic microcircuit card is a card ordinarily made from a rectangular sheet of plastic material which incorporates electronic microcircuits carried by a substrate and having contacts on the exterior for connecting the electronic microcircuits to an apparatus for processing cards. The electronic microcircuits may be intended for quite varied functions, such as for bank debiting and crediting operations, for allocating telephone message units and for confidential input into a classified system. Generally, the microcircuits comprise processing and/or memory circuits of variable complexity depending on their intended use. In practice, they are formed on as least one silicon microchip, currently known as an integrated circuit or chip. For convenience, the term "chip" will be used here.
The chip substrate contained in the card is typically a very thin sheet of plastic material including the elements for interconnection between the chip or chips and the contacts on the card. In a particular instance, the substrate has the card contact on one of its faces, while its other face has the chip or chips and their elements for interconnection with the contacts. All this comprises a device known as a contact assembly or package, which includes all the functional elements of the card; the term "package" will be used here. Accordingly, manufacturing the card merely comprises incorporating the package into the plastic material of the card, such that the contacts of the package are flush with one face of the card and that the chip and its circuits will be protected under the package by a minimum thickness of the plastic material making up the card. Assembling the package is done at present by inserting it into a cavity on the card, but it may also be done by being embedded in the plastic material of the card. The invention relates to the manufacture of a package of this kind.
A package of this type must first of all meet the usual conditions for the use of flexible portable cards intended for the general public. To do so, the package must have sufficient mechanical strength to protect the chip and its interconnection elements from the stresses the substrate undergoes when the card is being manipulated. Meanwhile, the package must be flexible enough to match its bending and twisting to the bending and twisting the card can ordinarily be subjected to, yet without damage to the chip, its elements for interconnection with the contacts, and the soldering between the elements and the chip.
A package of this type must also be able to be incorporated into a card having a thickness that meets ISO standards for credit cards, that is, 0.76 mm.+-.10%, while leaving a sufficient thickness of plastic material underneath the package to protect the chip and its interconnection elements. In terms of nominal values, it has been found in practice that the minimum thickness of the plastic material for protection purposes is about 0.20 mm, which leaves a thickness of only 0.56 mm for the package itself. In the package, a chip already occupies a thickness of 0.30 mm, which is obtained after machining down the original thickness of a standard chip. This leaves a possible thickness of only 0.26 mm for the assembly comprising the substrate, the contacts and the elements for connecting the terminals of the chip to the respective contacts of the package. In this assembly, as noted above, the support must have sufficient mechanical strength, which in practice requires that a thickness on the order of 0.1 mm. Finally, the contacts are generally formed beginning with a copper foil glued to the substrate and then engraved and covered with a layer of gold and at least one intermediate compatibility layer, of nickel, for example.
On the other hand, to be well suited to large-scale mass production of cards at low cost, the method for manufacturing a package must be simple and economical. A method of large-scale production of packages for cards is clearly of technical and commercial interest only if it involves a minimum of components and method steps. Simplicity becomes even more important considering that given the constraints on the thickness of a package, its mass production must also accommodate relatively wide ranges of tolerance for each element of the package. Simplicity thus becomes a fundamental and decisive factor in choosing one method among all the possible methods.
Finally, in practice, it has been found that for a great number of different specific services, the corresponding cards require the use of packages of relatively large width, on the order of 4 to 8 mm on a side, for example. Furthermore, any card can include only a low number of contacts, for example 8, in the case of a credit card. This limits the terminals of the corresponding chip to the same number, which may even be fewer in certain cases. On a large chip, the terminals can be spaced sufficiently far apart from one another to be connected directly to the respective contacts of the card. Thus for large chips there exists the possibility to further simplify the methods of manufacture and make it unnecessary to connect the terminals of the chip to the contacts of the package via connection elements such as wires and conductors that are either free or integral with the substrate.
Previous methods of manufacturing packages do not meet all the conditions described above, especially if the packages are intended for use on cards of standardized thickness.
A present method of manufacture of such packages is described in French Patent No. 2 439 478, corresponding to U.S. Pat. No. 4,264,917 to Ugon, and assigned to the assignee of the present invention. The method comprises piercing holes in the substrate, gluing a copper foil on one face of the substrate and engraving it and then gold-plating it to form the contacts of the package, metal-plating the holes, and then proceeding with the steps of applying and photoengraving a conductive layer on the other face of the substrate to form connection zones, and soldering the input/output elements of the integrated circuits carried on one or two chips onto these connection zones. In other words, this method utilizes the technology of metallized holes and of metallization on both faces of the substrate. Thus, not only is this method complicated, but the resultant package proves to be expensive and bulky.
A variant being used at present comprises making an opening in the substrate for gluing the chip directly onto part of the copper foil, which seals this opening. Advantageously, the corresponding package is slight in thickness because the thickness of the substrate is not added to that of the chip, and the portion of the copper underneath the chip promotes the removal of heat produced by the microcircuits and can be used to polarize a chip of the N-MOS type. However, connecting the chip directly to the thin outer sheet of copper foil has the disadvantage of making the chip more vulnerable to the pressure, torsion and bending that the general public may impose on flexible cards in the course of normal use. Furthermore, the manufacturing method remains complex and expensive, and it also requires that the chip terminals be connected by wires to the connection zones having the contacts.
An improvement of these manufacturing methods has been proposed in French Patent Nos. 2 439 322 and 2 439 438 also assigned to the assignee of the present invention. In this improvement, the ends of the conductive fingers of the chip are fixed directly to respective contacts in the holes which are pierced into the substrate, closed by the contacts, and at least partially filled with solder. Nevertheless, adding the solder to the holes is a very delicate step in the method. Furthermore, either before or after the soldering, an equally delicate operation of bending the conductive fingers accurately must be performed.
It should be noted that in addition to the defects peculiar to particular methods, there is another disadvantage common to all the previous methods described above. This is that all these methods require the presence of connection elements between the terminals of the chip and the contacts of the package. These connection elements are the connecting wires, or the conductive fingers of the chips, or the conductors formed beginning at a metal layer resting on the inside face of the substrate of the package, or a combination of these elements. The use of connecting wire represents the technology known as "wire bonding". Adding conductive fingers to the chips may be done by the technique of mounting chips on a tape, which is presently known as TAB, for tape automatic bonding. Finally, the formation of conductors on the substrate may be done by gluing and engraving of a sheet of copper foil, or by metal plating. In other words, all the previous methods necessarily include steps that involve expensive and sometimes delicate operations.